1. Field of the Invention
This invention relates to a method for controlling a compressor installation with at least one lubricated screw-type compressor element connected to a pressure vessel, which element is driven by an electric motor with continuously adjustable speed and to which a gas inlet conduit is connected, a gas outlet conduit is connected which is provided with an outlet valve, and a return conduit for lubricating liquid is connected which is provided with a closing valve, which screw-type compressor element compresses the gas supplied through the gas inlet conduit from a lower pressure to a higher, pre-set pressure, according to which method the speed of the motor is regulated in function of the compression pressure and therefore is diminished as the pressure in the pressure vessel obtains a certain value and, inasmuch as necessary, the motor is stopped in a programmed manner.
2. Discussion of the Related Art
In known methods, when the compressor element is working under load, as soon as the compression pressure and therefore the pressure in the pressure vessel have obtained a maximum value, then the motor is slowed down until a programmed stop command stops it possibly completely, and the screw-type compressor element comes to a standstill.
During the last stage of this slowing down, the outlet valve and the closing valve in the return conduit still are open in order to keep the temperature of the compressed air under control, and the number of revolutions is reduced to such an extent that in the screw-type compressor element a surplus of lubricating liquid is created by means of the return conduit and the closing valve. The quantity of lubricating liquid flowing back to the screw-type compressor element as long as the closing valve in the return conduit is open, in fact is determined by the pressure in the valve.
At low speeds of the screw-type compressor element, an accumulation of lubricating liquid in the screw-type compressor element may occur.
As this lubricating liquid is not compressible, then, as a result of hydraulic forces, at low speeds the load torque of the compressor element can increase considerably.
This also has as a consequence that, with the first subsequent start, the driving motor has to overcome a very high resistive torque, to which end particularly high electric powers are necessary.
This may cause a motor damage or failure of the drive or necessitates a corresponding over-dimensioning of the drive.
The invention has as an object a method for controlling a compressor installation which avoids the aforementioned and other disadvantages and which, when the speed-regulated, lubricated screw-type compressor element is running out, stopping and re-starting, avoids a surplus supply of lubricating liquid and, as a consequence thereof, the failure of the drive of said screw-type compressor element.
To this aim, according to the invention, the return flow of lubricating agent through the return conduit also is determined by a controlled valve which is provided with a calibrated opening and which is controlled by control means which, when the number of revolutions of the screw-type compressor element has dropped below a well-defined value, put the controlled valve into the position in which it restricts the return flow of lubricating agent to a flow through the calibrated opening and which remove this restriction when the number of revolutions surpasses a well-defined value.
As a result hereof, the load torque keeps its normal value and, therefore, starting problems are avoided.
The control of the controlled valve can take place by controlling a three-way valve in a conduit between the pressure vessel and a chamber inside the controlled valve, which three-way valve in one position connects said chamber to the pressure vessel, such that the pressure in the pressure vessel effects on the valve body of the controlled valve, and in a second position connects said chamber to the atmosphere, whereby the control means put the three-way valve into the first position when the number of revolutions of the motor drops below a well-defined value.
When the pressure in the pressure vessel obtains a well-defined value, preferably the motor and, therefore, the screw-type compressor element are stopped in two stages and during this stopping procedure, when the number of revolutions has dropped below a well-defined value, the controlled valve is put into the position whereby it restricts the return flow of lubricating agent, after which the motor further slows down until a programmed stop command stops it entirely and the screw-type compressor element comes to a standstill.
The invention also relates to a compressor installation which is particularly suited for the application of said method.
Thus, the invention relates to a compressor installation with at least one lubricated screw-type compressor element; a gas inlet conduit connected to this screw-type compressor element and a gas outlet conduit, provided with an outlet valve; an electric motor coupled to said screw-type compressor element with continuously adjust-able speed; a pressure vessel connected to the gas outlet conduit; a return conduit for lubricating agent, provided with a closing valve, between the pressure vessel and the interior side of the screw-type compressor element; and control means for controlling the speed of the motor in function of the compression pressure and to give a stop signal thereto in a programmed manner, and which is characterised in that in the return conduit, between the closing valve and the screw-type compressor element, a controlled valve is installed, with a calibrated opening which in one position closes off the return conduit, with the exception of the calibrated opening, and in another position allows a normal flow through the return conduit.
In known compressor installations, a closing valve indeed is present in the return conduit for the lubricating agent, but no additional controlled valve.